1. Field of the Invention
The invention relates to a multi-microphone capsule with a plurality of microphones disposed inside.
2. Description of the Related Art
FIG. 1 shows a conventional microphone capsule 100 that is used for various voice communication devices. The microphone capsule 100 includes an electret sensor 120 and a J-channel field effect transistor (J-FET) 140 that are mounted within a housing 110. The electret sensor 120 implements a single microphone. An electret is a dielectric material that has been permanently electrically charged or polarized. Incoming sound waves enter via a top opening 112 and are translated into mechanical vibrations upon contacting the electret sensor 120. The electret sensor 120 converts the sound vibrations into an electrical signal that varies in voltage amplitude and frequency corresponding to the original sound. The J-FET 140 receives and amplifies the electrical signal from the electret sensor 120 and provides an output signal. The J-FET 140 is mounted on a printed circuit board (PCB) 130 and is further coupled to external circuitry via openings 150 formed at the bottom of the housing 110.
FIG. 2 is a schematic diagram of the conventional microphone capsule 100 and an electronic unit 190. The electret sensor 120, for the microphone capsule 100, is modeled with a voltage source that generates an electrical signal based on the incoming sound. The J-FET 140 amplifies the electrical signal and provides the output signal to the electronic unit 190.
The electronic unit 190 includes a resistor 192, a capacitor 194, and an amplifier (Amp) 196. The resistor 192, coupled between a supply voltage (Vcc) and the drain of the J-FET 140, acts as the circuit load for the J-FET 140, and further provides bias current for the J-FET 140. The resistor 192 is typically a small value (e.g., 1 KΩ). The output signal from the drain of the J-FET 140 includes an alternating current (AC) portion for the desired audio signal and a direct current (DC) portion for the bias current for the J-FET 140. The capacitor 194 couples between the drain of the J-FET 140 and the input of the amplifier 196, performs AC coupling (or DC blocking), and passes the AC portion for the desired audio signal to the amplifier 196. The amplifier 196 amplifies the audio signal and provides an amplified signal to subsequent circuit blocks (not shown in FIG. 2).
As shown in FIG. 2, the microphone capsule 100 typically includes two electrical contacts for (1) the output signal, which includes the desired audio signal and bias current, and (2) circuit ground.
The microphone capsule 100 includes a single microphone that is implemented with a single electret sensor 120. Depending on the design of the microphone capsule 100, this single microphone may be an omni-directional microphone or a uni-directional microphone.
FIG. 3A shows a beam pattern for an omni-directional microphone, which is roughly equally sensitive to sound coming in from all directions. An omni-directional microphone may be created for the microphone capsule 100 by not having any openings for sound at the bottom of the housing 110, not shown in FIG. 1.
FIG. 3B shows a beam pattern for a uni-directional microphone, which is more sensitive to sound coming in from a particular direction (typically the front side). FIG. 3B shows a cardioid microphone, which is a common type of uni-directional microphone, having a beam pattern resembling the shape of a heart. A uni-directional microphone may be created for the microphone capsule 100 by forming openings 150 for sound at the bottom of the housing 110 (as shown in FIG. 1). Incoming sound waves then enter the microphone capsule 100 via both the top and bottom openings 112 and 150. The sound waves received via the bottom openings 150 are canceled by the sound waves received via the top opening 112, thereby creating low sensitivity for the bottom side.
FIGS. 3A and 3B show two beam patterns for a microphone. Other beam patterns, such as a bi-directional (or dipole) pattern, may also be formed with different placement of acoustic openings.